The present invention relates to coating solutions and fuser member layers, including methods for preparing the fuser member layers. More specifically, the present invention relates to coating solutions and fuser member layers prepared using the coating solutions, wherein the coating solutions comprise a fluoropolymer such as a fluoroelastomer, a crosslinking agent, and a polar solvent. Such fuser members can be used in electrostatographic reproducing apparatuses to fuse toner to a substrate.
In a typical electrostatographic reproducing apparatus, a light image of an original to be copied is recorded in the form of an electrostatic latent image upon a photosensitive member and the latent image is subsequently rendered visible by the application of electroscopic thermoplastic resin particles which are commonly referred to as toner. The visible toner image is then in a loose powdered form and can be easily disturbed or destroyed. The toner image is usually fixed or fused upon a support which may be the photosensitive member itself or other support sheet such as plain paper.
The use of thermal energy for fixing toner images onto a support member is well known. To fuse electroscopic toner material onto a support surface permanently by heat, it is usually necessary to elevate the temperature of the toner material to a point at which the constituents of the toner material coalesce and become tacky. This heating causes the toner to flow to some extent into the fibers or pores of the support member. Thereafter, as the toner material cools, solidification of the toner material causes the toner material to be firmly bonded to the support.
Typically, the thermoplastic resin particles are fused to the substrate by heating to a temperature of between about 90.degree. C. to about 200.degree. C. or higher depending upon the softening range of the particular resin used in the toner. It is undesirable, however, to increase the temperature of the substrate substantially higher than about 250.degree. C. because of the tendency of the substrate to discolor or convert into fire at such elevated temperatures, particularly when the substrate is paper.
Several approaches to thermal fusing of electroscopic toner images have been described. These methods include providing the application of heat and pressure substantially concurrently by various means, a roll pair maintained in pressure contact, a belt member in pressure contact with a roll, a belt member in pressure contact with a heater, and the like. Heat may be applied by heating one or both of the rolls, plate members, or belt members. The fusing of the toner particles takes place when the proper combination of heat, pressure and contact time are provided. The balancing of these parameters to enable the fusing of the toner particles is well known in the art, and can be adjusted to suit particular machines or process conditions.
It is important in the fusing process that minimal or no offset of the toner particles from the support to the fuser member take place during normal operations. Toner particles offset onto the fuser member may subsequently transfer to other parts of the machine or onto the support in subsequent copying cycles, thus increasing the background or interfering with the material being copied there. The referred to "hot offset" occurs when the temperature of the toner is increased to a point where the toner particles liquefy and a splitting of the molten toner takes place during the fusing operation with a portion remaining on the fuser member. The hot offset temperature or degradation of the hot offset temperature is a measure of the release property of the fuser, and accordingly it is desired to provide a fusing surface which has a low surface energy to provide the necessary release. To ensure and maintain good release properties of the fuser, it has become customary to apply release agents to the fuser roll during the fusing operation. Typically, these materials are applied as thin films of, for example, silicone oils to prevent toner offset.
The process for the preparation of such fuser members is important in maintaining desired fuser life. Known processes for providing surfaces of fuser members include two typical methods which are dipping of the substrate into a bath of coating solution or spraying the periphery of the substrate with the coating material. However, recently, Applicants have developed a process which involves dripping material spirally over a horizontally rotating cylinder. Generally, in this new flow coating method, the coating is applied to the substrate by rotating the substrate in a horizontal position about a longitudinal axis and applying the coating from an applicator to the substrate in a spiral pattern in a controlled amount so that substantially all the coating that exits the applicator adheres to the substrate. For specific details of an embodiment of the flow coating method, attention is directed to commonly assigned Attorney Docket Number D/96036, U.S. Pat. application Ser. No. 08/672,493 filed Jun. 26, 1996, entitled "Flow Coating Process for Manufacture of Polymeric Printer and Belt Components," the disclosure of which is hereby incorporated by reference in its entirety.
However, not all coatings are compatible with the new flow coating method. Specifically, only materials which can be completely dissolved in a solvent can be flow coated. Further, it is desirable that the material have the ability to stay dissolved during the entire flow coating process which may take up to approximately 8 hours or longer, and must stay dissolved during the manufacturing period which may be up to several days. Good results are not obtained with materials which tend to coagulate or crystallize within the time period required for flow coating, which may be on the order of about 8 hours and for production manufacturing, may be on the order of a few days, for example, from about 1 to about 4 days. It is important to use a material capable of being flow coated for an increased amount of time to enable flow coating in a manufacturing and production environment. It is very costly to periodically shut down a manufacturing line and change the solution delivery system. If the coating does not have the desired properties, the assembly line may need to be shut down often, for example, every hour or every few hours in order to clean the delivery line of coagulated or crystallized material. Therefore, it is desirable to use a material which has good flow coating properties in order to allow for manufacturing to continue for a long period of time, for example several days, without occurring the above problems in the procedure.
It is also desirable that the coating be slow drying to avoid trapping solvent in the under layers which tends to cause bubbles and solvent "pops." Bubbles result from trapped air in the coating which results in non-uniformity of coating and or surface defects. Solvent "pops" are defined as trapped air or solvent voids that rupture resulting in crater-like structures causing non-uniform coated areas or surface defects. In either case, these defects can act as initiation sites for adhesion failures.
Moreover, useful materials for the flow coating process must possess the ability to flow in a manner which allows for the entire roll to be coated. Therefore, it is desirable that the material possess a desired viscosity which allows it to flow over the entire surface of the member being coated. Along with these properties, it is desirable that the material to be coated possess a balance between viscosity and percent solids. Similarly, it is important that the coating material have the ability to completely dissolve in a solvent in order to prevent precipitation of the material which can lead to non-uniform flow coating, and imperfections in the final flow coated surface.
The balance between viscosity and percent solids is important to enable sufficient build rates which impact throughput and work in process. Build rates are defined as the thickness of a material that can be coated per unit time. The thickness of the material should allow for a balance between maintaining thickness uniformity and avoiding solvent "pops" and air bubbles. Throughput in the process is the number of units that are processed per unit time. Work in process (WIP) is the number of units currently in any one of the process stages from beginning to end. The objective is to maximize the build rate and reduce the throughput time and work in process.
Many materials known to be useful for outer coatings of a fuser member, such as, for example, silicone rubbers, fluoropolymers and fluoroelastomers, possess some of the above qualities necessary for flow coating. However, problems result once the fluoroelastomer is dissolved in a solvent and crosslinking or curing agents are added. For example, the curative or crosslinking agents tend to precipitate within minutes after addition to the solvent solutions. The precipitate causes numerous problems such as clogging the filters and pumps used in the flow coating process. Further, the entire fuser member cannot be coated due to early precipitation of the curing and/or crosslinking agent. In addition, early precipitation may lead to non-uniform flow coating and imperfections in the final flow coated surface.
U.S. Pat. Nos. 5,338,587 and 5,366,772 disclose mixing a fluoroelastomer with a nucleophilic curative and methyl ethyl ketone solvent and spray coating the solution onto the roll.
There exists a need for flow coating solutions for flow coating fuser member layers which possess the qualities necessary for sufficient flow coating including providing slow solidification following flow coating, possessing the ability to remain in solution without precipitation, and providing a sufficient balance between flowability, viscosity and percentage solids.